1. Field of the Invention
The present invention relates to a display device, and more particularly, to a liquid crystal display device and a method of fabricating the same. Although the present invention is suitable for a wide scope of applications, it is particularly suitable for forming a liquid crystal display panel device having a uniform cell gap.
2. Discussion of the Related Art
A liquid crystal display device has characteristics of full color realization, low voltage operation, low power consumption, thinness, lightness in weight, and high image quality. Thus, its applications have been diversified from monitors for electronic watches, calculators, notebook computers, personal computers and TV, and gauge monitors for aviation, personal digital assistants (PDA), and mobile stations.
In fabricating the liquid crystal display device, it is necessary to form a liquid crystal layer between lower and upper substrates. A method for forming the liquid crystal layer is largely classified into two methods, such as a liquid crystal injection method and a dispensing method. In case of a small sized panel, the liquid crystal layer is formed by injecting liquid crystals between the lower and upper substrates in a vacuum condition through an injection inlet after bonding the lower and upper substrates to each other. If a size of the panel is large, such a vacuum injection method has a problem in that it takes a long time to inject the liquid crystal. Accordingly, in case of a large sized panel, the liquid crystal is dispensed on one of the lower and upper substrates, and then the substrates are bonded to each other to form the liquid crystal layer.
In addition, spacers are used for maintaining a uniform cell gap between the lower and upper substrates. There are two kinds of spacers: the one is a ball spacer being scattered on the substrate, and the other is a column spacer being attached to the substrate. With a trend of the large sized panel, the column spacer has been mostly used since the ball spacer has limitations in maintaining a uniform cell gap in the large sized panel. At this time, materials for maintaining the uniform cell gap are used for a sealant for bonding the substrates to each other as well as for the substrates. The materials may be made of a glass fiber of which thickness maintains the cell gap between the lower and upper substrates.
After bonding the lower and upper substrates to each other, the sealant is hardened so as to fix the bonded state of the substrates. At this time, the sealant is classified into a thermo-hardening sealant being hardened by heat and a photo-hardening sealant being hardened by UV-ray. A seal mixture of an epoxy resin and a hardener is usually used as a thermo-hardening sealant. If a thermal process is performed to the seal mixture, the hardener is chemically combined with the epoxy resin, and the epoxy resin activated by the hardener is chemically combined with another epoxy resin. Accordingly, the seal mixture is polymerized so that adhesion of the seal mixture is improved.
After forming the liquid crystal layer between the lower and upper substrates, the bonded substrates are treated in a press for one to two hours so as to completely bond the lower and upper substrates to each other. The photo-hardening sealant is a seal mixture of a hardener and acrylate resin such as epoxy acrylate resin or urethane acrylate resin. If UV-ray is irradiated to the sealant, the hardener becomes a radical, and then the radical acts with the acrylate resin, so that the acrylate resin is activated. Accordingly, the seal mixture is polymerized so that the adhesion of the seal mixture is improved. After forming the liquid crystal layer between the lower and upper substrates, the UV-ray is irradiated to the bonded substrates so as to harden the sealant between the lower and upper substrates for several seconds.
A related art liquid crystal display device and a method of fabricating the same will be explained with reference to the accompanying drawings. FIG. 1 illustrates a plane view of a liquid crystal display device according to a related art liquid crystal dispensing method. FIGS. 2A to 2G illustrate fabrication process of a liquid crystal display device according to the related art liquid crystal dispensing method.
Referring to FIG. 1, a plurality of gate lines and data lines are formed on a first substrate 100 to cross each other and define a plurality of pixel regions 112. A plurality of thin film transistors (TFTs) are formed at each crossing point of the plurality of gate and data lines, and a plurality of pixel electrodes are formed in the pixel regions for being electrically connected to drain electrodes of the TFTs. Thereafter, silver (Ag) dots are formed at predetermined portions of the first substrate 100 for applying a voltage to a common electrode of a second substrate, and liquid crystal is dispensed on an active region.
Then, a black matrix layer, a color filter layer, and the common electrode are formed on the second substrate 150, and column spacers 150 are formed on portions of the second substrate corresponding to the gate and data lines of the first substrate 100 except for the pixel regions 112 of the first substrate 100. Upon formation of the column spacers, a photo-hardening sealant 110 including a glass fiber 125 is formed on the second substrate 150. The first and second substrates 100 and 150 are bonded to each other, and then UV-ray is irradiated to the photo-hardening sealant 110 including the glass fiber, thereby hardening the photo-hardening sealant 110. Furthermore, the column spacers 105 are formed within the active region 120 to maintain a cell gap on the entire panel of the liquid crystal display device. The cell gap is uniformly maintained at the circumferential region of the active region 120 by the thickness of the glass fiber 125 included in the photo-hardening sealant 110.
In this case, the photo-hardening sealant 110 including the glass fiber 125 is formed as follows. First, the photo-hardening sealant 110 is mixed with the glass fiber 125 at a ratio of 100 to 1 in weight for about ten minutes. A seal mixture of the photo-hardening sealant and the glass fiber is stirred to remove bubbles for two hours under a vacuum condition. The seal mixture without any bubbles is hermetically maintained in a cylinder at a temperature below 0 □. Then, the seal mixture and cylinder are maintained at the ambient temperature for about three hours or more. When the glass fiber 125 is mixed with the photo-hardening sealant 110 having high viscosity, gas bubbles are generated. Even though the gas bubbles are removed under a vacuum condition in a bubble remover, the gas bubbles are not completely removed since the high viscosity sealant is used to form the seal mixture. Accordingly, the photo-hardening sealant is unevenly formed, and spots appear on the panel due to the gas bubbles. Also, external moisture may penetrate into the liquid crystal display device through the sealant due to an out-gassing of the gas bubbles inside the sealant, thereby degrading a picture quality.
A method of fabricating a related art liquid crystal display device will be explained in detail. FIGS. 2A to 2G illustrate fabrication process of a liquid crystal display device according to a related art liquid crystal dispensing method. In FIGS. 2A to 2G, a plurality of liquid crystal display panel regions 200 are formed on one substrate, and then are cut into a plurality of unit panels.
Referring to FIG. 2A, a thin film transistor (TFT) array including gate and data lines (not shown) is formed at each liquid crystal display panel region 200 on a first substrate 100, and silver (Ag) dots are formed with a constant distance in the peripheral region of each liquid crystal display panel region 200 on the first substrate 100. Then, liquid crystal 103 is dispensed on each liquid crystal display panel region 200. The silver (Ag) dots are formed for applying a voltage to a common electrode formed on a second substrate 150.
As shown in FIG. 2B, column spacers 105 are formed on a black matrix layer (not shown) of the second substrate 150, on which a color filter array is formed at each liquid crystal display panel region.
Then, a photo-hardening sealant 110 including a glass fiber 125 is formed in the peripheral region of each liquid crystal display panel region on the second substrate 150, as shown in FIG. 2C.
Referring to FIG. 2D, the silver (Ag) dots 101 and the liquid crystals 103 are formed on the first substrate 100, and the column spacer 105 and the sealant 110 are formed on the second substrate 150. The first and second substrates 100 and 150 are loaded to a bonding apparatus being controlled to be under a vacuum condition. The surface of the second substrate 150, on which the sealant 110 is deposited, faces down, and then is fixed to an upper stage 170 that moves along the Z-axis direction (i.e., vertical direction). Also, the first substrate 100 is fixed to a lower stage 160 that moves along the XY-axis direction (i.e., horizontal direction). At this time, the silver (Ag) dots 101 are disposed outside the photo-hardening sealant 110.
As shown in FIG. 2E, the second substrate 150 fixed to the upper stage 170 and the first substrate fixed to the lower stage 160 are aligned, and a chamber of the bonding apparatus becomes in a vacuum condition. The upper stage 170 moves down to bond the first and second substrates 100 and 150 to each other.
Referring to FIG. 2F, gas or dry air is provided to the chamber of the bonding apparatus under a vacuum condition so as to equalize a pressure of the chamber to the ambient pressure. Accordingly, the first and second substrates bonded by the sealant 110 are maintained under a vacuum condition, and the circumferential region of the substrates is under the ambient pressure. Thus, the first and second substrates are pressed to each other caused by a pressure difference between the inside pressure of the substrates and the ambient pressure. Simultaneously, the liquid crystals 103 spread out on the entire panel between the first and second substrates, thereby forming a liquid crystal layer 103a. 
Thereafter, the first and second substrates 100 and 150 bonded to each other are loaded on a quartz stage 180, shown in FIG. 2G. The UV-ray is irradiated to the bonded substrates from the bottom of the first substrate 100, thereby hardening the photo-hardening sealant 110 including the glass fiber 125 with the UV-ray.
However, the related art liquid crystal display device according to the liquid crystal dispensing method and the method of fabricating the same have the following disadvantages.
The photo-hardening sealant is mixed with the glass fiber, so that the gas bubbles are generated. Accordingly, spots appear on the panel due to the remaining gas bubbles after bonding the first and second substrates to each other. Also, it is difficult to maintain a uniform cell gap between the substrates in that the photo-hardening sealant is unevenly formed.
Furthermore, the external moisture may penetrate into the liquid crystal layer through the sealant due to an out-gassing of the gas bubbles inside the sealant, thereby degrading a picture quality.